Planning a deck requires careful calculation of materials to avoid costly trips back to the builder's merchant or wasted materials. Getting the quantities right for boards, joists, and fixings is essential for both DIY and professional installations.
This guide explains how to calculate decking materials accurately, compare timber versus composite options, and understand UK planning requirements for garden decking projects.
Decking Board Calculation
Add 10% for waste, cuts, and future repairs
Common Board Sizes
| Type | Width | Length | Coverage/board |
|---|---|---|---|
| Timber (standard) | 145mm | 3.6m | 0.52 m² |
| Timber (wide) | 195mm | 3.6m | 0.70 m² |
| Composite | 140mm | 3.6m | 0.50 m² |
| Composite (wide) | 190mm | 3.6m | 0.68 m² |
Example: 4m × 3m Deck
Area: 12 m²
Using 145mm × 3.6m boards (0.52 m² each):
Boards = 12 ÷ 0.52 = 23 boards
Plus 10% waste: 23 × 1.1 = 26 boards
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Joist Calculation
| Board Type | Max Joist Spacing |
|---|---|
| Softwood (25mm) | 400mm centres |
| Softwood (32mm) | 500mm centres |
| Hardwood | 450mm centres |
| Composite | 400mm centres |
Plus 2 outer joists and noggins between at 1m intervals
Materials List Example (4m × 3m)
| Item | Quantity | Est. Cost |
|---|---|---|
| Decking boards (3.6m) | 26 | £160-260 |
| Joists (4.8m × 47mm × 150mm) | 9 | £90-120 |
| Decking screws (box) | 2 boxes | £30-40 |
| Joist hangers | 18 | £20-30 |
| Weed membrane | 15 m² | £15-25 |
Timber vs Composite
| Factor | Timber | Composite |
|---|---|---|
| Cost (per m²) | £25-50 | £60-120 |
| Lifespan | 10-15 years | 25-30 years |
| Maintenance | Annual treatment | Occasional wash |
| DIY friendly | Yes | Needs special clips |
| Appearance | Natural | Uniform |
UK Decking Costs 2025
| Material Type | Material Cost/m² | Installed Cost/m² |
|---|---|---|
| Pressure-treated softwood | £25-40 | £80-120 |
| Hardwood (e.g., ipe, balau) | £80-150 | £150-250 |
| Budget composite | £40-60 | £100-150 |
| Premium composite | £80-120 | £150-220 |
| Capped composite | £100-150 | £180-280 |
Cost Example: 4m × 3m Deck (12 m²)
Timber DIY: £25-40 × 12 = £300-480 materials
Timber installed: £80-120 × 12 = £960-1,440 total
Composite DIY: £60-100 × 12 = £720-1,200 materials
Composite installed: £120-180 × 12 = £1,440-2,160 total
Timber Types for Decking
| Type | Pros | Cons |
|---|---|---|
| Pressure-treated softwood | Affordable, widely available | Needs annual treatment, splinters |
| Thermally modified | Stable, no chemicals | More expensive, can crack |
| European larch | Natural oils, attractive | Medium cost, greys with age |
| Cedar | Naturally rot-resistant | Soft, dents easily |
| Hardwood (ipe/teak) | Very durable, 25+ years | Expensive, hard to work with |
Composite Decking Brands UK
Popular composite decking brands available in the UK:
- Trex: Premium US brand, 25-year warranty
- Millboard: UK brand, wood-look resin
- Eva-Last: Bamboo-plastic composite
- Cladco: Budget-friendly UK supplier
- TimberTech: Premium with wide colour range
- Hyperion: Mid-range capped composite
Deck Building Checklist
- Measure area: Length × width, note any angles
- Check regulations: Planning and building control
- Clear ground: Remove turf, level area
- Lay weed membrane: Prevents growth through deck
- Install frame: Joists at correct spacing (400-500mm)
- Fix boards: Gap between boards for drainage (5-8mm)
- Add finishing: Fascia boards, edge trim
Maintenance Schedule
| Task | Timber | Composite |
|---|---|---|
| Sweep/clean | Monthly | Monthly |
| Wash (jet wash) | Twice yearly | Twice yearly |
| Oil/stain | Annually | Not needed |
| Check fixings | Annually | Annually |
| Sand smooth | Every 2-3 years | Not needed |
Timber vs Composite Decking: UK Costs and Considerations
Choosing between timber and composite decking is one of the most significant decisions for any UK garden project. Both materials have distinct advantages, and the right choice depends on your budget, maintenance preferences, and how you intend to use your outdoor space.
Softwood Timber Decking
Pressure-treated softwood, typically Scandinavian pine or spruce, remains the most popular and affordable decking choice in the UK. Boards typically cost between 3 and 8 pounds per linear metre, making it the budget-friendly option. Softwood decking is easy to cut and work with, which reduces installation costs. However, it requires regular maintenance including annual cleaning, treatment with decking oil or stain every one to two years, and inspection for rot or insect damage. In the damp UK climate, untreated softwood can deteriorate within five to eight years. With proper maintenance, you can expect a lifespan of 10 to 15 years.
Hardwood Timber Decking
Hardwood options such as oak, iroko, and balau offer superior durability and a premium appearance. Prices range from 8 to 20 pounds per linear metre depending on the species. Hardwoods are naturally more resistant to rot and insect damage, often lasting 25 to 40 years with minimal maintenance. They weather to an attractive silver-grey if left untreated, or can be oiled to maintain their original colour. When sourcing hardwood, look for FSC or PEFC certification to ensure sustainable forestry practices.
Composite Decking
Composite decking, made from a blend of recycled wood fibres and plastic polymers, has grown significantly in popularity across the UK. Boards cost between 12 and 35 pounds per linear metre, with premium brands commanding higher prices. The key advantage is minimal maintenance: composites do not require staining, oiling, or treatment, and most manufacturers offer warranties of 20 to 25 years. They resist rot, splinters, and insect damage. However, composite boards can become slippery when wet unless they have a textured or grooved surface, and they can feel warmer underfoot in direct sunlight compared to timber.
Joist Spacing and Subframe Requirements
Regardless of your chosen decking material, the subframe is critical to the deck's longevity and structural integrity. Standard joist spacing in the UK is 400mm centres for timber decking and 300 to 350mm centres for composite boards, which are generally less rigid. Joists should be a minimum of 47mm x 100mm treated softwood or, for elevated decks, 47mm x 150mm. Always ensure a slight fall of approximately 1 in 80 (about 12mm per metre) away from the house to allow rainwater drainage. In the UK's wet climate, adequate airflow beneath the deck is essential to prevent moisture buildup, so maintain a minimum ground clearance of 50mm.
Planning Permission and Building Regulations
Most garden decking in England does not require planning permission, as it falls under permitted development rights. However, there are important conditions. The decking, together with any other extensions, outbuildings, and additions, must not cover more than 50% of the total garden area. The platform must not be more than 300mm above the existing ground level to qualify as permitted development. If the decking is at the front of the house or on a listed building, planning permission is required. For properties in conservation areas, national parks, or Areas of Outstanding Natural Beauty, additional restrictions may apply. Scotland, Wales, and Northern Ireland each have their own permitted development rules, so always check with your local planning authority.
UK Weather Considerations for Decking
The British climate presents unique challenges for outdoor decking. Frequent rainfall means moisture management is paramount: ensure adequate drainage, use stainless steel or coated screws to prevent rust staining, and apply anti-slip treatments to timber surfaces. Freeze-thaw cycles during winter can cause timber to crack and composite boards to contract slightly, so always leave expansion gaps of 5 to 8mm between composite boards. Algae and moss growth is common on shaded decks in the UK's damp conditions. Regular brushing with a stiff broom and annual application of a proprietary deck cleaner will keep surfaces safe and looking their best. For areas prone to heavy shade, consider grooved composite boards which provide better grip in wet conditions.
Frequently Asked Questions About UK Decking
How much does it cost to build a deck in the UK?
A typical 15 square metre softwood deck costs between 1,500 and 3,000 pounds for materials and professional installation. Composite decking for the same area ranges from 3,000 to 6,000 pounds. These figures include the subframe, fixings, and basic labour. Additional features such as steps, balustrades, or built-in lighting will increase costs.
Do I need building regulations approval for decking?
Standard ground-level decking generally does not require building regulations approval. However, raised decks over 600mm high or decks attached to the house structure may need to comply with building regulations, particularly regarding structural loading and safety balustrades. Any balustrade must be at least 1,100mm high for decks above 600mm.
How long should UK decking last?
Treated softwood decking lasts 10 to 15 years with regular maintenance, hardwood decking lasts 25 to 40 years, and composite decking typically lasts 20 to 30 years. Lifespan is heavily influenced by maintenance, exposure to moisture, and the quality of the subframe construction.
What maintenance schedule should I follow?
For timber decking, sweep regularly, deep clean with a decking cleaner each spring, and apply oil or stain annually. Check for loose screws, signs of rot, and structural integrity each year. Composite decking requires only periodic sweeping and an occasional wash with soapy water. Regardless of material, clear fallen leaves promptly as they trap moisture and promote algae growth.
UK Building Regulations and Decking Standards
While most ground-level garden decking in the UK falls under permitted development rights, there are important regulatory considerations that every homeowner should understand. Under Part A of the Building Regulations (Structure), any deck that is raised more than 600mm above ground level may require building regulations approval. This includes structural calculations to ensure the subframe can safely support the expected loads, which the regulations define as a minimum of 1.5 kN per square metre for domestic use.
The Timber Decking Association (TDA), the UK's leading trade body for decking, publishes comprehensive installation guidelines that are widely regarded as industry best practice. Their Decking Design Guide specifies minimum joist sizes, maximum spans, and fixing requirements for both timber and composite decking systems. Following TDA guidelines helps ensure your deck meets insurance requirements, as poorly constructed decking can void home insurance cover if it causes damage to the property.
In terms of environmental considerations, the UK decking market has shifted significantly towards sustainably sourced materials. FSC and PEFC certification schemes ensure that timber comes from responsibly managed forests. The UK imports approximately 80 percent of its softwood timber, primarily from Scandinavia and the Baltic states, where sustainable forestry practices are well established. Composite decking manufacturers increasingly use recycled materials, with some brands containing up to 95 percent recycled content. This aligns with the UK government's 25 Year Environment Plan, which promotes sustainable construction practices.
For properties in flood-risk areas, the Environment Agency recommends that decking and other hard surfaces should allow rainwater to drain through to the ground beneath. Permeable decking with adequate gaps between boards (typically 5 to 8mm) helps meet Sustainable Drainage Systems (SuDS) requirements, which many local authorities now mandate for new developments under Schedule 3 of the Flood and Water Management Act 2010.
Practical Tips for UK Decking Projects
- Order materials in spring for summer installation. UK builders' merchants and decking suppliers typically experience peak demand from April to July. Ordering your materials in February or March often secures better prices and ensures stock availability, particularly for popular composite brands which can have lead times of four to six weeks during the busy season.
- Always use stainless steel or coated fixings. The UK's damp climate makes corrosion a serious concern. Standard steel screws will rust and stain the decking within months. Invest in A2 or A4 stainless steel decking screws, or use the proprietary coated fixings recommended by your decking manufacturer. The additional cost is minimal compared to replacing stained boards.
- Check your home insurance before building. Some UK home insurance policies require notification of structural changes to the property, including raised decking. Failing to inform your insurer could invalidate your cover. Most standard garden decking under 300mm high does not affect premiums, but it is worth confirming with your provider before you begin construction.